Silicon nitride belongs to the family of technical ceramics. The special properties of silicon nitride make it an especially suitable material for the manufacture of articles intended for use at high temperatures, typically from 1000.degree. to 1200.degree. C. The articles manufactured from silicon nitride are obtained by high-temperature sintering, under a nitrogen atmosphere, of a powder of silicon nitride in .alpha. form, to which an additive that promotes densification is traditionally added. The mechanical properties of articles produced from silicon nitride are mechanical strength, resistance to thermal shock, hardness and high wear resistance, and these articles are hence widely used in every type of industry. As examples, silicon nitride is used for the production of automobile rocker arms, turbocompressor rotors, precombustion chambers for diesel engines in the automobile industry, ballbearings, nozzles, extrusion dies and cutting tools.
The known processes for the synthesis of silicon nitride on an industrial scale are as follows:
i) Direct nitriding of silicon: EQU 3 Si+2 N.sub.2 .fwdarw.Si.sub.3 N.sub.4 PA1 ii) Thermal decomposition of a silicon imide, for example according to the equations: EQU 3 Si(NH).sub.2 .fwdarw.Si.sub.3 N.sub.4 +2 NH.sub.3 EQU 3 Si(NH.sub.2).sub.4 .fwdarw.Si.sub.3 N.sub.4 +8 NH.sub.3 PA1 iii) Carbonitriding of silica: EQU 3 SiO.sub.2 +6 C+2 N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6 CO
This reaction is highly exothermic and difficult to control, and yields coarse powders which hence have to be ground before sintering is carried out.
This reaction yields a powder which is very fine but nevertheless amorphous, and which has to be crystallized at a temperature of 1300.degree. to 1500.degree. C.
This process possesses the major advantage of employing inexpensive starting materials and of not yielding any effluent other than CO, which is readily burnt. Nevertheless, carbonitriding possesses drawbacks. The reaction temperature must be perfectly controlled. Above 1470.degree. C., there are risks of formation of SiC. Below 1400.degree. C., there are risks of formation of a poorly crystallized product containing a large amount of oxygen. During the process, there is competition between the formation of two crystalline forms of Si.sub.3 N.sub.4, the .alpha. and .beta. forms. The .alpha. form is the desired form, since this is the one which yields sintered bodies possessing the above mentioned properties. Furthermore, the particle size of the powder is critical for the carrying out of sintering; a perfectly controlled particle size is hence necessary. Since the process is carried out under a stream of nitrogen, carry-over of materials often takes place. This carry-over is due to the formation of an intermediate, SiO gas, abbreviated to SiO.sub.(g), which can either disproportionate at a temperature of 1000.degree. to 1200.degree. C. to SiO.sub.2 and Si, resulting in fouling of the plant, or lead to the formation of rod-shaped crystals of Si.sub.3 N.sub.4(w), termed "whiskers", which are unsuitable for sintering, probably according to the reaction: EQU 3 SiO.sub.g +3 CO.sub.g +2 N.sub.2 .fwdarw.Si.sub.3 N.sub.4w +3 CO.sub.2
In order to solve the above mentioned problems many processes have been proposed.
The addition of seed crystals, as described in Patents U.S. Pat. No. 4,428,916 and FR-A-2,388,763, has made it possible to favor the production of Si.sub.3 N.sub.4 powders in .alpha. form and a controlled particle size. Control of the particle size is, however, only partially obtained, and the problems of formation of by-products, in particular of the gaseous kind, remain.
The addition of additives, as described in Patents GB-A 1,028,977 and EP-A 131,894, facilitates carrying out the carbonitriding process, in particular as regards formation of the .alpha. form of Si.sub.3 N.sub.4 and particle size, as above, but also enables coarser-sized starting materials to be used. Furthermore, this addition of additive has a catalytic action on the formation of silicon nitride. Thus, EP-A-131,894 describes and claims a process for the manufacture of fine powder of .alpha.-Si.sub.3 N.sub.4 by carbonitriding a mixture, prepared by adding at least one of the following additives, namely (a) a mixture of at least one of the elements Be, Mg, Ca, Sr, Ge, Sn, Ti and Hf and compounds of these elements with 0.01 to 1 part by weight of silicon nitride powder, or (b) a mixture of Zr and compounds of the latter with 0 to 1 part by weight of silicon nitride powder, in a total quantity of 0.001 to 0.1 part by weight, calculated in terms of elemental weight(s) of Be, Mg, Ca, Sr, Ge, Sn, Ti, Hf and Zr, and 1 part or less by weight of silicon nitride powder, to 1 part by weight of silicon oxide powder and to 0.4-parts by weight of carbon powder.
Unfortunately, the problem of formation of by-products is not solved, and is even made worse, since gaseous compounds of the additives form and condense on the cold parts of the reactor, leading to silicon nitride powders whose final content of these additives is not controlled. The sintering of these products is hence made more difficult. Moreover, the technique described in EP-A-131,894 is a batch process (operating batchwise) which cannot be used in continuous fashion on account of the accumulation of the elements mentioned above.